With the increasingly high performance, diversification and size and weight reduction of electronic devices in recent years, there has been an increase in devices designed with transparent touch panels (touch sensors) mounted across the full screens of display devices such as liquid crystal displays. An increasing number of devices also have transparent touch panels that allow visibility and selectability of characters, symbols and images displayed on the display device, with switching of various functions of the device being performed by operation of the transparent touch panel. Touch panels are used not only in large-size electronic devices such as personal computers and television sets, but also in a wide variety of display devices including those of car navigation systems, cellular phones, miniature electronic devices such as electronic dictionaries, and OA⋅FA devices, wherein the touch panels are provided with electrodes made of transparent conductive electrode materials. Known transparent conductive electrode materials include ITO (Indium-Tin-Oxide), indium oxide and tin oxide, which are mainly used as electrode materials for liquid crystal display unit substrates because of their high visible light transmittance.
Existing touch panel systems that are currently being implemented include resistance film systems, optics systems, pressure systems, electrostatic capacitive systems, electromagnetic wave guide systems, image recognition systems, vibration detection systems and ultrasonic systems, the greatest increase in use, in recent years, being that of electrostatic capacitive touch panels. With an electrostatic capacitive touch panel, contacting the touch input screen with the fingertip (which is a conductor) creates electrostatic capacitive coupling between the fingertip and the conductive film, forming a condenser. The electrostatic capacitive touch panel notes changes in electrical charge at the location of contact with the fingertip, thereby detecting the coordinates. In particular, projection-type electrostatic capacitive touch panels have high operability as they allow detection of multiple points with the fingertip, permitting complex selection to be made, and for this reason they are being increasingly utilized as input devices for display surfaces on devices with small-size displays such as cellular phones and portable music players. In a projection-type electrostatic capacitive touch panel it is common to form a two-layer structure from a plurality of X electrodes and a plurality of Y electrodes perpendicular to the X electrodes, in order to present two-dimensional coordinates with an X-axis and a Y-axis, with ITO generally being used as the electrode material.
Incidentally, a touch panel has a frame region in which it is unable to detect touch locations, and therefore one important way of increasing product value is to narrow the area of the frame region. The frame region must have metal wiring for transmission of signals detected at the touch locations, and the width of the metal wiring must be narrowed in order to achieve a narrower frame area. Copper is most commonly used as the metal wiring.
Such touch panels, however, when contacted with fingertips, can become infiltrated with corrosive components such as moisture and salts inside their sensing regions. When corrosive components infiltrate the interior of a touch panel, the metal wiring becomes corroded and electrical resistance between the electrodes and driving circuit increases, or wire breakage may occur in some cases.
Moreover, the touch panel itself is subjected to physical load during the production steps for the touch panel. Particularly when a protective film is provided on a flexible display panel, the load on the protective film increases as the panel is curved, and this tends to result in cracking.
A demand therefore exists for a protective film that reduces corrosion of metal wiring, that has high adhesiveness with flexible display panels, and that is flexible sufficient to withstand curving of panels.
Patent Literatures (PTLs) 1 and 2 propose means for meeting this demand. In PTL 1, the protective film is used for the purpose of minimizing corrosion of the copper wiring used for transmission of touch location detection signals, but the purpose of flex resistance is not mentioned. PTL 2 describes how toughness can be imparted to a material by combination with a specific crosslinking agent, but mentions only the elastic modulus, with no mention regarding any connection with crack resistance.
Protective films designed to protect circuit board surfaces or pattern circuits on flexible display panels include not only the protective films mentioned above but also various types of photosensitive material films such as photosensitive cover lay films and photosensitive dry film resists, etc., which are used depending on the intended purpose.
Most photosensitive cover lay films and photosensitive dry film resists are polyimide-based, acrylic or epoxy-based films, since properties such as heat resistance, chemical resistance and flex resistance are required for the cured films.
Epoxy-based films have long been employed, but while they exhibit excellent heat resistance and chemical resistance, they also lack flex resistance.
As an example of an acrylic film, PTL 3 proposes a dry film resist to be used for fabrication of a printed circuit board. In PTL 3, excellent flexibility is obtained for the cured resist, but nothing is mentioned in regard to the performance required as a permanent material for protecting electrodes or metal wiring (for example, minimizing corrosion of the copper wiring, or regarding the thermomechanical strength of the material).
A polyimide-based film is proposed in PTL 4, for example. In PTL 4, the heat resistance, chemical resistance and flex resistance of the cured film is excellent, but there may be concerns generation of gas regarding imidation reaction during thermosetting or during deblocking of blocked isocyanates.